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Medical retina
Published in Mostafa Khalil, Omar Kouli, The Duke Elder Exam of Ophthalmology, 2019
The retinal pigment epithelium (RPE) is composed of a single layer of cuboidal epithelial cells containing melanosomes and has many functions: Absorbs light and prevents the scattering of light within the eye.Replenishes the molecules needed for phototransduction.Contains a blood-retinal barrier, which provides a selectively permeable membrane to supply nutrients to the photoreceptors and maintain homeostasis. The blood-retinal barrier is maintained by the zonulae occludentes.Phagocytosis of photoreceptor outer segment membranes.Transport and storage of metabolites and vitamins.
Regulators of Signal Transduction: Families of GTP-Binding Proteins
Published in Robert I. Glazer, Developments in Cancer Chemotherapy, 2019
The heterotrimeric G-proteins serve regulatory roles in activation (Gs) and inhibition (Gi) of hormone-sensitive adenylate cyclase activities, phototransduction in the retina (Gt), sensory transduction in olfaction, and ion gating (Mg2+, Ca2+, and K+ channels). G-proteins have clearly been implicated in a number of other secondary messenger systems, for example, in the activation of phospholipases C and A2. The term G-protein is herein used to describe a family of four distinct proteins, termed Gs, Gi Go, and Gt (otherwise known as transducin). Members of this family share a number of features including being heterotrimeric with structurally related α subunits (Mr = 39 to 52 kDa), functionally interchangeable γ subunits (Mr = 35 to 36 kDa), and smaller γ subunits (Mr = 6 to 10 kDa). These subunits are probably present in the holoprotein at a molar ratio of 1:1:1.
SBA Answers and Explanations
Published in Vivian A. Elwell, Jonathan M. Fishman, Rajat Chowdhury, SBAs for the MRCS Part A, 2018
Vivian A. Elwell, Jonathan M. Fishman, Rajat Chowdhury
The visual pathway may be summarized as follows: Photoreceptors (rods, cones) within the retina convert light energy into electrical impulses (phototransduction)This is transmitted to ganglion cells, directly via bipolar cells, or indirectly via horizontal and amacrine cellsGanglion cells are the output cells of the retina. Axons from ganglion cells converge at the optic disc (blind spot) and travel in the optic nerveIncomplete decussation occurs at the optic chiasm; those from the nasal half of each retina (corresponding to the temporal halves of the visual field) cross over (decussate), while those from the temporal halves of each retina stay on the same sideThe optic tracts synapse in the various layers of the lateral geniculate nucleus of the thalamus before being relayed to the primary visual cortex in the occipital lobe via the optic radiation
Representations of the olfactory bulb and tracts in images of the medieval cell doctrine
Published in Journal of the History of the Neurosciences, 2022
For some of the special senses (vision, audition, and gustation), the illustrated linkage between the sensory organ and the sensus communis is straightforward, even if erroneous and simplistic. With vision, for example, an afferent pathway (usually represented as a straight line) typically connects the eye (and, in particular, usually the pupil, where light enters the eye) to the sensus communis. There was, of course, then no conception that the retina is responsible for phototransduction. Similar lines connect the nose (olfaction), ears (audition), and the tongue (gustation) with the sensus communis. Uncommonly, some of the special senses were illustrated as having connections with the middle or posterior ventricle of the brain: For example, German anatomist Johann Dryander showed connections between the pupils and two of the ventricles (the anterior and middle ventricles), and between the tongue and the posterior or third ventricle (Dryandrum 1537).
Neuro-Ophthalmic Literature Review
Published in Neuro-Ophthalmology, 2022
David A. Bellows, Noel C.Y. Chan, John J. Chen, Hui-Chen Cheng, Peter W. MacIntosh, Michael S. Vaphiades, Xiaojun Zhang
Sildenafil citrate, a selective oral phosphodiesterase 5 inhibitor, is a widely used drug for erectile dysfunction that acts by elevating cyclic guanosine monophosphate levels and causing smooth muscle relaxation. It also has 10% activity against phosphodiesterase 6, a key enzyme in the phototransduction cascade in the retina. Recent ocular imaging developments have further revealed the influence of sildenafil on ocular haemodynamics, particularly choroidal perfusion. Choroidal thickness is increased, and choroidal perfusion is also enhanced by autoregulatory mechanisms that are further dependent on age and microvascular abnormalities. Studies demonstrating high intraocular pressure via a “parallel pathway” from increased choroidal volume and blood flow to the ciliary body have challenged previous concepts. Another new observation is the effect of sildenafil on bipolar cells and cyclic-nucleotide gated channels. The authors discuss potential deleterious effects (central serous chorioretinopathy, glaucoma, ischaemic optic neuropathy, and risks to recessive carriers of retinitis pigmentosa), potential beneficial effects (amelioration of choroidal ischaemia, prevention of thickening of Bruch membrane, and promotion of recovery of the ellipsoid zone) in macular degeneration, as well as potential drug interactions of sildenafil.
Exploring the effects of large-area dorsal skin irradiation on locomotor activity and plasm melatonin level in C3H/He mice
Published in Chronobiology International, 2021
Xuewei Fan, Zeqing Chen, Wenqi Li, Haokuan Qin, Shijie Huang, Zhicheng Lu, Yinghua Li, Muqing Liu
Nonetheless, whether the skin has the ability to transmit this time information into body and even affect the central clock is still unclear. Some researchers have conducted relevant studies in human which are classified as extraocular phototransduction. Campbell and Murphy et al. found that intense light irradiation on popliteal region (behind the knee) could cause phase shift in core body temperature and dim light melatonin onset (DLMO), and even enhance rapid eye movement sleep (REM) (Campbell and Murphy 1998; Murphy and Campbell 2001). However, this finding was questioned by other researchers. Wright et al., in particular, obtained the opposite results in an almost identical experiments (Wright and Czeisler 2002). Recently, Wulff-Abramsson et al. observed that blindfolded people had different EEG signals when exposed to red, green or blue light (Wulff-Abramsson et al. 2019). This result suggested that light signals from skin appeared to be transmitted to the central nervous system and resolved by it.